GB1492626A - Monitor system for sensing discrete points - Google Patents

Abstract

1492626 Selective signalling WALT DISNEY PRODUCTIONS 3 Oct 1975 40599/75 Heading G4H In a monitoring system for a large number of devices each of which can assume a normal state and an off-normal state, a central scanner 947 addresses groups of 256 devices in turn, each group when addressed responding with a first word indicating whether all the devices in the group are in their normal states or whether any is in its off-normal state, and only if any device in the group is in its off-normal state, the devices of that group are scanned by a local scanner 965, 16 at a time, to derive 16 words each including 16 data bits, each data bit indicating the state of a respective device. In Fig. 1, the computer 946 normally plays no part in the proceedings, but if the first word from an addressed group indicates a device in an off-normal condition, all 17 words from that group are sent to the computer which, by reference to a stored table, decides what corrective action is needed and transmits addressed control signals to appropriate control devices 966. The central scanner 947 includes an address counter (992). An interrogation signal comprises 18 bits, bits 1-8 are all 1's and define the signal as an interrogation signal, bits 10-17 are the address of the interrogated group, bits 9 and 18 are parity bits. Each interrogation signal is sent twice and a remote terminal unit 960 performs parity and sync checks on the signals. If more than a given number of errors occurs, unit 960 selects the second occurrence of the interrogation signal and informs the computer. All interrogation signals go to all the local scanners 965 but only that one with the correct address responds, by transmitting a first 18-bit word including 8 address bits (which are compared in scanner 947 as a check), 2 parity bits and bits denoting whether all the devices are in their normal states and whether a piggy-back group exists for that address. If all devices are normal and no piggy-back group is present, the central scanner address counter is incremented by 1 to address the next group, If a piggy back exists, it is interrogated next by changing the most significant bit of the address. If an offnormal device is signalled, the local scanning is carried out, 16 18-bit resulting each including 2 parity bits, all 17 words from the group being routed to the computer. Details of the central and local scanners are given.